In a body of cars and the like, used is a suspension integrated with a shock absorber for reducing the vibration of the body caused by road surface roughness, the shaking thereof which occurs in quick acceleration or sudden braking, and the like. The structure of the shock absorber is based on a cylindrical structure that utilizes the resistance of oil to flow. Specifically, used is the structure having small holes bored in a hydraulic piston. In the slide part between the cylinder and the piston rod, a bush is provided to be a bearing. In general, the bush is formed of bronze.
During expansion and contraction movement, a shock absorber may receive a great lateral force, and in the case, friction is generated in a bush. The generation of friction may be a factor to worsen the riding comfort performance, and therefore it is desired to reduce the friction.
Heretofore, for reducing the friction generated in a bush, it has been investigated to blend a higher fatty acid such as stearic acid, isostearic acid or the like in a lubricating oil composition for a shock absorber. However, though a linear higher fatty acid such as stearic acid or the like could realize friction reduction for bronze, the corrosiveness thereof to bronze is high and the wear-resistant properties could not be therefore bettered, and further, the solubility in a base oil is low, which may often cause precipitates. On the other hand, a branched higher fatty acid such as isostearic acid or the like could have a high solubility in a mineral oil but could not sufficiently realize friction reduction for bronze, and further has a problem in that the wear-resistant properties could not be bettered. Namely, heretofore, suitable friction reduction for bronze-made bushes could not be realized by the use of a higher fatty acid.
In addition, heretofore known is a lubricating oil composition for a shock absorber, in which a phosphorus acid ester as an extreme-pressure agent is blended and a secondary amine is further blended, for example, as shown in PTL 1. However, this lubricating oil composition could not still sufficiently reduce the friction to bronze-made bushes.
Further, for example, in PTL 2, it is known to blend a tertiary amine in a lubricating oil composition for continuously-variable transmissions, along with an extreme-pressure agent composed of a phosphorus acid ester and a metal compound such as a metal sulfonate or the like. However, when this lubricating oil composition is used in a shock absorber without modifying, the friction coefficient for bronze-made bushes could not still be sufficiently reduced.